Like gr5 said: this is a cooling problem: the hot nozzle keeps sitting on same spot on top of the model, so it can't cool down and solidify. This is a limitation of this printing-concept.
I often print small models that have this problem. Then I print multiple at once, so one can cool while the other is printing. Or I print a dummy tower next to the real model. Ideally, the dummy should have the complementary surface area as the real model, so that the total area per layer is constant. Brutal changes in layer area show up as horizontal lines on small models.
Printing slow and cool also helps.
I am no fan of moving away the nozzle without extruding: then the filament keeps sitting in the nozzle for a longer time, and gets a different viscosity. This also shows up as horizontal deformations in the model. And the nozzle may leak while sitting and waiting aside, causing blobs.
A dummy tower reduces the effect, but does not eliminate it:
Theoretical model of a dummy with complementary shape:
Part of a real design:
Tiny models, each ca. 20mm high, printed at different speeds and temperatures. Printing tiny models on their back -if the model allows it - also increases layer cooling time:
Edited by geert_2
That's a common problem. I usually solve that by printing two (or even 5) of the same object.
Or I print a tower right next to the part. This allows the previous layer to cool a little while it prints the tower (or other object). Otherwise the nozzle never leaves the puddle/blob and so it can't solidify.
Another solution is called "lift head". Enable that feature and set the minimum layer time to something like 3 or 5 seconds. Then when it gets to these tiny layers it will move the head away for a few seconds to cool. The problem is you are just trading one problem for another - while the head is "lifted" it will slowly (over 2 seconds) extrude a tiny little sausage-string and then when it continues it will glue that into the side of the part and you get these annoying hairs.
I prefer the tower/2 parts solution instead.
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